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Missiles are guided weapons designed to deliver a destructive payload to a target. They can be launched from various platforms, including aircraft, ships, and ground-based launchers. Some missiles are designed to hit stationary targets, while others are intended to engage moving targets, such as aircraft or cruise missiles.
One of the main challenges in intercepting maneuverable aerial targets is their ability to perform evasive maneuvers. Such targets can change their direction and altitude rapidly, making them difficult to track and hit with a missile. To overcome this challenge, modern missile systems often use advanced guidance systems that can adjust the missile’s trajectory in real-time to keep up with the target’s movements.
A solid propellant Divert Attitude Control System (DACS) is a quick reaction propulsion system providing control over positions of missiles, satellites, and spacecraft. Regarding missiles, whether a ballistic measure, counter measure or defensive measure, DACS allows for interception of its target with greater accuracy and reliability.
DACS are comprised of a large number of propellant-based rocket motors or thrusters. The guidance and control subsystem will provide firing commands to individual DACS motors to orient the missile onto the target.
There are a number of factors that contribute to accurately hitting a target from any weapon system. There is the stability of the weapon, the proper aim point, projectile speed, adjustment for humidity, wind, elevation and other environments that influence the flight of the projectile. Finally, there is the target itself; its size, its distance, its surrounding environments and maneuverability.
Generally, a target is more particularly characterized by its motion, i.e. speed, direction, maneuverability, trajectory. The target can be hit by a missile guided according to a guidance law (command to line of sight, proportional navigation), which brings the missile all the closer to the target as the latter moves in a slow and regular manner. In the case of an aerial target, the closer the explosion is to the objective, the smaller the warhead can be, for a given probability of destruction of said objective. Any error at the end of the flight must be compensated for by a final maneuver of missile piloting to the target. Thus, the higher the maneuverability and evasion performances of the target, the higher the lateral acceleration, particularly within the interception area, and the shorter the response times, should be.
“Hit-to-kill” application is where the fired projectile attempts to hit a high-speed, moving target, sometimes referred to as “hitting a bullet with a bullet”. To increase the probability of hit in this application and due to extremely high closing speeds, a Divert Attitude Control System (DACS) is required to provide the final course correction to the missile to hit the target.
Usually, the missile uses “PAF” to manoeuvre. That is to say, it uses aerodynamic force, requiring certain aerodynamic effectiveness to result in a moment that turns the vehicle. In order to use this aerodynamic effectiveness, you’ve either got to have massive, badass fins (resulting in lots of drag and weight), or lots of speed. In a missile of standard aerodynamic control achieved by angle of attack pickup, the time constant related to the aerodynamic response is always great, in the order of several tenths of second. The control devices provoking such angle of attack pickup are either of the aerodynamic type or controls in the jet of the main propulsive device, or else, through lateral auxiliary jets from the main propulsive device or independent elements.
One can certainly utilize a highly performing guidance law, such as that applied to known arms systems, but efficiency thereof depends on the knowledge of the time remaining before interception of the objective, and this, in a jamming environment, cannot be evaluated with the accuracy required for the intended purpose. Therefore, it is advisable to compensate for insufficiency of the guidance according to the above criteria, by increasing maneuverability of the defensive missile, i.e. increasing the load factor, and above all, reducing the response time of said defensive missile.
In the design of new weapons, a typical offencing target, representing a threat particularly difficult to destroy consists of a extremely maneuverable supersonic missile performing the final approach in skimming flight or going into a steep dive. In this case indeed, the belated discovery of the hostile missile requires the earliest possible neutralization to assure security of the site to be protected. For the time being, assault from such aggressors is difficult to oppose by means of the known defensive systems. A defensive missile with a conventional aerodynamic control guided on proportional navigation cannot intercept such aggressors unless it is equipped with a very extensive warhead.
DACS is one part of the “PIF-PAF” control technique – namely, the PIF. PIF and PAF are two methods used to manoeuvre missiles. PIF-PAF is actually an acronym, PIF: Pilotage Inertiel en Force (Inertial Steering with Force) and PAF: Pilotage Aeronautique en Force (Aerodynamic Steering with force). One of them says “Use a force to steer the body”, and the other says “Use a force to change the aerodynamic properties to steer”.
The PIF, or, DACS is a non-conventional method which consists of blowing air or exhaust gases with a component that is perpendicular to the instantaneous flight path of the missile. This causes the missile to turn. The advantage of this method is that it allows the missile to turn in regions where there is low aerodynamic effectiveness – read, “low(er) speed”. Furthermore, a response time of only a few hundreths of second can be obtained by utilization of forces substantially passing through the center of gravity, and such forces can be acquired aerodynamically, or by lateral jets. In this case, there is little or no aerodynamic angle of attack pickup, but rather direct displacement of the center of gravity. Such a known mode of control called force control and designated hereinafter as PIF essentially supplies very high quickness of response.
ASTER Missiles
ASTER missiles are currently in service on board the latest naval vessels of France, Italy, and the United Kingdom. Equipped with an active RF seeker, the ASTER missile is autonomously guided and capable of simultaneously targeting and engaging multiple threats enabling it to counter saturated attacks. The ASTER missile family comprises ASTER 15 the short-medium range version and ASTER 30 the long-range version. The shot-range version, ASTER 15 has a maximum speed of Mach 3, a maximum range of 30 km and an interception altitude of 13 km. The long-range version, ASTER 30 can reach speeds of Mach 4.5 while reaching altitudes of 30 km with a maximum range of 120 km and can perform aerial maneuvers greater than 60 Gs giving it a very high degree of maneuverability.
Highly maneuvering and agile ASTER missiles use a direct thrust vector control system called “PIF-PAF” which is intentionally located at the missile’s center of gravity to maximize responsiveness to prevent ruptures under high-g maneuvers during trajectory corrections. During the flyout toward the target, ASTER can perform 90-degree trajectory changes. PAF (Pilotage Aeronautique en Force) uses long chord wings and fins for strong aerodynamic control authority. This is supplemented by PIF (Pilotage Intertiel en Force) consisting of four gas jets acting through the centre of gravity of the missile. The narrow jets are integrated into the fins to avoid disruption of the airflow and are capable of generating rapid lateral movements to keep the missile on target. There are other missiles that use thrust vectoring but what may be unique is the use of the central PIF jets to apply a force to the body in the opposite direction to manoeuvre-induced G-force to prevent the missile rupturing under stress. Engineered to cope with trajectory corrections up to 60G, there is certainly no aircraft and probably no missile that could out-manoeuvre a locked-on Aster in its terminal phase.
DARPA Glidebreaker and Next Generation Interceptor (NGI)
In April 2022, DARPA published a notice inviting industry proposals for Glide Breaker Phase 2 . DARPA began Glide Breaker in fiscal year 2019 with a two-phase plan: first, focus on a single, critical, long-lead technology with applicability to a variety of interceptor concepts and designs; second, develop additional component technologies.
DAPPA’s fiscal year 2023 budget request seeks $18.2 million for Glide Breaker, up from $7 million a year in both FY-21 and FY-22. “Glide Breaker intends to advance the United States’ means to counter hypersonic vehicles by enabling intercept of hypersonic threats in glide phase utilizing an interceptor launched from an Aegis MK-41 Vertical Launch System,” states DARPA’s recent notice. “In particular, the focus of Glide Breaker is on enabling a divert and attitude control system (DACS) propelled KV capable of intercepting hypersonic threats during glide phase.”
Glide Breaker Phase 2 seeks to advance understanding of this phenomena in order “to enable a DACS-propelled KV to intercept threats during glide phase in the presence of [jet interaction] effects. If successful, the results of Phase 2 will provide the foundation for a future program of record interceptor,” according to the agency.
Separately, the Missile Defense Agency is funding research on an alternative technology with potential application for countering hypersonic glide vehicles. In 2019, MDA’s advanced technology program executive office awarded Aerojet Rocketdyne a nearly $19 million contract for Axial Upper Stage technology risk reduction as part of the agency’s mandate to develop a hypersonic defense system.
“DARPA’s Glide Breaker program is a divert and attitude control system and is complementary to MDA’s Axial Upper Stage propulsion system effort,” MDA spokeswoman Heather Reed Cavaliere told Inside Defense. “Although both are applicable to advanced threats, they are different technologies for different specific applications.”
Cavaliere said MDA and DARPA routinely exchange technical data and provide subject-matter experts to support these and other projects through collaborative independent verification and validation; acquisition support; solicitation review; and specific technology area expertise
Protecting America from an incoming intercontinental ballistic missile attack is a complex task involving radars, space sensors, interceptors and a robust command and control system that must work as one. Our nation’s interceptor program is one of the most critical systems serving to protect everyone living in the United States every second of every day.
To support the Next Generation Interceptor (NGI) program – as the prime contractor, Northrop Grumman has strategically partnered with Raytheon Technologies and assembled a premier team that will provide a highly advanced capability while focused on affordability, transparency, rapid development, lean manufacturing and a low-risk schedule. Building on decades of experience as a prime contractor for major defense programs, Northrop Grumman will provide a comprehensive NGI solution that supports operational deployment on a rapid timeline to meet the United States’ urgent national security needs.
Raytheon Missiles & Defense, Aerojet Rocketdyne complete risk reduction efforts on key NGI propulsion tech, reported in Dec 2021
Raytheon Missiles & Defense, a Raytheon Technologies business, completed a demonstration to reduce risk to the overall technical baseline for the Next Generation Interceptor. The demonstration was an early engineering test of the thruster valve and nozzle on the liquid propellant divert and attitude control system (DACS) designed by Aerojet Rocketdyne.
Raytheon is strategically partnered with Northrop Grumman on the Missile Defense Agency (MDA) program to rapidly design, develop and test an advanced interceptor solution to defend the nation against complex long-range threats.
“We know from experience the criticality of tackling the most challenging aspects of a payload program first,” said Scott Alexander, executive director, Integrated Missile Defense Solutions at Raytheon Missiles & Defense. “Early risk reduction work allows us to make data-informed design and testing decisions, and ultimately that’s going to help us get this critical capability to the nation as soon as possible.”
The test’s results will inform Aerojet Rocketdyne’s DACS design and ensure it is on a robust path for further risk reduction and development activities. Aerojet Rocketdyne has been a supplier for Raytheon Missiles & Defense across numerous missile defense programs for 20 years, jointly developing and providing the DACS for key systems deployed today.
Northrop Grumman and Raytheon Missiles & Defense currently provide the interceptor booster, kill vehicle, ground systems, fire control and engagement coordination for the country’s Ground-based Midcourse Defense (GMD) system. Aerojet Rocketdyne provides industry-leading kill vehicle and kinetic warhead propulsion to all of the MDA programs currently in production, including the GMD system.
“We’re proud to partner with the Northrop Grumman and Raytheon team on Next Generation Interceptor to enable the defense of our nation with DACS that are reliable and affordable,” said Eileen P. Drake, Aerojet Rocketdyne CEO and president. “Aerojet Rocketdyne’s proven DACS has performed exceptionally on GMD system flight tests, and we are excited to build on our decades of missile defense experience with ground-breaking DACS technology and a highly skilled workforce.”
Aerojet successfully tests lightweight Kill Vehicle Divert Thruster
Aerojet, a GenCorp (NYSE: GY) company, announced today that the company has successfully tested a bi-propellant liquid rocket divert thruster applicable to the Early Intercept Divert and Attitude Control System (DACS). Both altitude and sea level tests were conducted as a risk reduction effort for the Early Intercept DACS.
Early Intercept is being developed as part of the Missile Defense Agency’s Ballistic Missile Defense System to rapidly respond to the growing ballistic missile threat. The successful test highlights Aerojet’s leading role in providing reliable propulsion solutions for both MDA and U.S. military services. “This test is a further testament to the innovation and reliability of Aerojet’s propulsion products,” said Aerojet’s Vice President of Defense Systems, Dick Bregard.
Aerojet supports the next generation of sea- and land-based U.S. missile defense capabilities by providing reliable, low-risk propulsion technologies. In addition to the current production of the Exo-Atmospheric Vehicle (EKV) DACS, the MK 72 and MK 104 rocket motors and Terminal High Altitude Air Defense (THAAD) boosters, Aerojet is developing the solid rocket Throttleable Divert and Attitude Control Systems (TDACS) for SM-3 Blocks IB and IIA, the planned missile variant upgrades to the current SM-3 Block IA.
DACS requirement for Ballistic Missile Defence
Ballistic Missile Defense kinetic warheads (KWs) utilize a DACS to maneuver the KW to intercept ballistic missile threats. A solid propellant is used to provide safe storage aboard Navy vessels. Current solid propellant DACS have performance limitations relative to liquid propellant DACS with respect to operating time, divert distance and energy management (on/off capability) and mass. These limitations result in reduced missile performance.
Mission requirements for fast (high burnout velocity) interceptors require a light weight KW. To meet these evolving requirements, DACS technology will require improvements in high-temperature, lightweight materials; innovative propellants; thrust control techniques; and performance characterization. Specific areas of interest include 1. High temperature, light weight materials: Development/demonstration of light weight structural insulator materials that can perform as rigid insulation to reduce component weight and volume, can replace multi layered components by performing as pressure vessels. These materials should be able to maintain dimensional stability under high thermal loads (>3000F) and thermal shock (70F-3000F within 500 ms) conditions under varying operating pressures (15psi -1500 psi).
2. Propellants: Demonstrate innovative propellants that demonstrate enhanced controllability. 3. Thrust control techniques: develop innovative technologies for thrust control, including on/off valve technology, light weight actuators, and electronic control techniques. 4. Innovative SDACS architectures: Devise innovate technologies for SDACS architectures that improves performance (energy management) while reducing overall SDACS mass to achieve performance similar to liquid DACS.
MDA called for Innovative concepts to place a Divert and Attitude Control System (DACS) onto a hypersonic gun launched projectile. This will enable low cost projectile based ballistic missile defense systems.
Current DACS technology is based on compressed cold gas, solid fuel, or liquid fuel thruster systems. Recent advancements in gun technology, in particular railgun technology, have opened the possibility to projectile based kill vehicles. Current railgun technology is sufficient to place a kill vehicle into position for exoatmospheric intercepts assuming the kill vehicle components are capable of surviving the gun launch and short hypersonic flight out of the atmosphere. MDA is looking for DACS concepts for a notional railgun projectile that is based upon a 40 mm diameter long rod finned projectile. The DACS must be balanced symmetrically about the centerline and may be split into a forward and rear section along the projectile.
The DACS concepts may be cold gas, solid fueled hot gas, or a combination and capable of diverting a projectile with an initial mass of 12 kg to 2 km/sec with a total DACS weigh less than 7 kg. The DACS must be capable of surviving a 30,000 g gun launch and survive the hypersonic transit at Mach 6 through the atmosphere without appreciable impact upon the aerodynamics of the projectile while inside the atmosphere.
Advanced Divert and Attitude Control System Materials for Liquid/Gel DACS Systems
Current DACS thrust chamber designs employ columbium components with a maximum operating temperature of 2600 °F. The proposed C-SiC component design has a significantly higher operation temperature (3700 °F) with extended operational duty times. The ceramic matrix composite design proposed here will have a thrust capability of 1000 lbf (pound force), also significantly greater than the current state of the art design. The added thrust capability will enable higher performance advanced designs, applicable to GMD / KEI missile programs employing liquid/gel DACS systems, including THAAD. Development of missile control systems incorporating C-SiC composite materials and novel joining technology holds promise to significantly reduce system cost and complexity while also increasing the thrust-to-weight ratio as compared to refractory metal based system concepts.
Fiber Materials, Inc., teamed with Aerojet and Plasma Processes, Inc., will complete the design and development leading to fabrication and demonstration of a ceramic matrix composite thrust chamber and requisite joining technologies for an advanced liquid/gel divert and attitude control (LDACS) application. The proposed Phase II development program will utilize a non-eroding material based on carbon fiber reinforced carbon silicon carbide (C-SiC).
References and Resources also include:
https://www.raytheonmissilesanddefense.com/news/2021/12/02/rmd-aerojet-rocketdyne
